As Dominick mentioned recently, I have started to help working on the Thinktecture.IdentityModel security library. My first contribution (which will be ongoing) is to maintain a .NET 4.0 compatible version (while Dom works on the 4.5 version). The claims based part of this will continue to use Windows Identity Foundation while the 4.5 version will use the new claims APIs built into .NET 4.5. I think there are enough people that will be on 4.0 for a while that this effort is worthwhile. There’s more than just claims support in there – there’s JWT and SWT token support, WebAPI authorization helpers, cookie protection helpers and more.
Here is the 4.0 GitHub repository.